1. Field of the Invention
The present invention relates to an organic electroluminescent device that converts electric energy into light and emits the light.
2. Description of the Related Art
Since organic electroluminescent devices (which may hereinafter be called “organic EL devices”) are capable of emitting light of high luminance at low voltage, they have been actively researched and developed. Organic electroluminescent devices have a pair of electrodes and an organic layer therebetween and utilize, for light emission, energy of the exciton generated as a result of recombination of electrons injected from the cathode and holes injected from the anode in the organic layer.
The devices have recently had improved efficiency by using phosphorescent materials. There are disclosed inventions relating to organic electroluminescent devices that use a compound having an N-phenylcarbazole skeleton between an organic light emitting layer and a cathode to improve the adhesion with the cathode, improve the long-term storage stability, and reduce dark spots (refer to, for example, JP-A-8-88083 and JP-A-8-60144).
JP-A-2008-085079 and JP-A-2003-335754 disclose inventions relating to organic electroluminescent devices using, a light emitting layer thereof, compounds having N-phenylcarbazole skeletons represented by the following formula, respectively:

The devices using the above compounds have however insufficient durability and in addition, they need further improvement in drive voltage.